Primary Chain Tensioner Fix

This fix was originally published in Joe Goffin's Aluminator Gibtec Build Thread

OK guys here they are and some pics of how they were made, some additional photos similar to Joe's about how the disassembled tensioners look and where they go as you reassemble.

I started with a 1 ft length of 3/4" 095 wall, seamless 4130 tubing because that was the closest that McMaster had to the size i needed. This is the tubing from McMaster. Aircraft Spruce will be less expensive than McMaster for the same tube;

I started to clean up the end with Scotch-brite™ and then decided it would be easier in the lathe so this is how it looked after a few seconds and Scotch-brite™;

The next job was to face off the end to make it square and chamfer the edges prior to parting off a piece. This is the parting off operation, you want the finished spacer to be 0.200" long.

After the parting off I broke the sharp edge next to the cut off and the job was complete. Repeat this one more time and you have two pieces. Total time was about 30 minutes plus clean up.

The spacers fit below the existing plunger as Joe has already indicated and prevent the chain tensioner from allowing the chain to go totally slack when the engine is shut down. This is a pic of how they fit into the tensioner assembly;

When you use the spacers the ratcheting plungers are no longer required.

There are a number of components in each assembly and as luck would have it they are different side to side. As a result I highly recommend you do one tensioner at a time and when you are done place it in a poly bag with all it's internals. Here is an exploded pic of most of the internals;

The right tensioner piston has a lubricating hole drilled in its top center. This piston will have a black plastic metering disc that goes into the inside top of the piston. The left tensioner piston will have no lubricating hole and no metering disc.

The ratcheting arm that we normally cut teeth off of is no longer required and can be discarded when you use the spacers beneath the tensioner piston. The metering valve you see in the pic below goes into the bottom of each piston well and is used to meter the volume of oil from the main galley that is fed to each piston to maintain chain guide pressure against the slack side of the primary drive chain.

If you happen to remove it in the modification process be sure to reinstall it prior to final assembly of the tensioner. The metering disc goes in with the pictured side facing the movable tensioning piston.

That's about it. This is not a particularly complex mod but it is a very good mod to do. The chain stretch at high engine speed or from a two step will cause the ratchet to extend on an unmodified tensioner and maintain tension on the primary drive chain as if it were stretched the way it was at high engine speeds — even after you shut off the engine.

That primary drive chain tension will squeeze the oil out of the #1 cam bearing saddle. The next time you start your engine you will have metal to metal contact until oil pressure can be restored to that journal. Repeated performances like this will eventually scar both the cam journal and the bearing saddle in the head. The next act in this unhappy show is the seizure of the #1 cam journal in the head. This is accompanied by the breakage of the primary drive chain and all the collateral damage the engine is capable of heaping upon your wallet.